Field of the Invention
The present invention relates to the field of flexible, rechargeable lithium-ion (Li-ion) batteries. In particular, the present invention relates to a printing or spray deposition method for preparing a supported flexible electrode that does not require the use of a synthetic polymer binder, or of organic solvents, or of plasticizers, and a method for manufacturing a lithium-ion battery comprising at least one such supported flexible electrode, easy to assemble and having good electrochemical performance.
Description of Related Art
Flexible Li-ion batteries can be used, like all conventional Li-ion batteries, in numerous devices that comprise portable equipment, such as notably mobile telephones, computers and light tools, or heavier equipment such as two-wheeled (bicycles, mopeds) or four-wheeled (electric or hybrid motor vehicles) means of transport. In general, flexible batteries may be used in all applications where it is desirable that the battery should be able to deform or bend, for example in order to fill empty spaces in hybrid or electric cars or for supplying flexible electronic devices other than in all the conventional applications of rigid Li-ion batteries.
A conventional lithium-ion (Li-ion) battery comprises at least one negative electrode (anode) and at least one positive electrode (cathode), between which there is a solid electrolyte or a separator impregnated with a liquid electrolyte. The liquid electrolyte consists for example of a lithium salt in solution in a solvent selected to optimize ion transport and dissociation. In particular, in a lithium-ion battery, each of the electrodes generally comprises a current collector (metal substrate), on which a composite is deposited that comprises a material that is active with respect to lithium, a polymer that performs the role of binder (for example a vinylidene fluoride (PVdF) copolymer), an agent conferring electron conductivity (for example carbon black) and a solvent.
During operation of the battery, lithium ions pass from one of the electrodes to the other through the electrolyte. During discharge of the battery, an amount of lithium reacts with the positive electrode active material from the electrolyte, and an equivalent amount is introduced into the electrolyte from the negative electrode active material, the lithium concentration thus remaining constant in the electrolyte. The insertion of lithium into the positive electrode is compensated by supply of electrons from the negative electrode via an external circuit. During charging, these phenomena take place in reverse.
The operation of flexible Li-ion batteries is the same as that described above for conventional Li-ion batteries. However, to obtain a flexible or foldable battery, it is necessary to develop, in addition, electrodes having not only good conductivity, but also in which the layer of active material has strong adhesion to the substrate, which makes it possible to avoid the appearance of cracks, or even detachment of the active material after bending the battery.
Various methods for manufacturing flexible electrodes have been proposed in the literature. In particular, in patent application FR 2 981 206 A1, self-supported anodes with improved flexibility were manufactured by filtration of an aqueous paste obtained by dispersing a mixture of solid particles comprising powdered graphite and refined cellulose fibres in an aqueous phase, on a filter cloth. This method uses environment-friendly raw materials and allows Li-ion batteries to be made that have good electrochemical performance. However, the filtration step requires said mixture of solid particles to represent only 0.02 to 5 wt % of the total weight of the aqueous paste. This step therefore involves the use of large volumes of aqueous phase, making industrialization of said method more complex. Above 5 wt % of mixture of solid particles in the aqueous paste, the filtration step becomes slower, leading to higher production costs. Moreover, according to the results presented, it seems that at least 10% of refined cellulose fibres is necessary to obtain anodes possessing good mechanical properties while maintaining sufficient conductivity.
The manufacture of flexible electrodes of the carbon nanotube (CNT)/Li4Ti5O12 (LTO) or carbon nanotube (CNT)/LiCoO2 (LCO) type by a coating process has also been proposed [ACS nano, 2010, 4, 10, 5843-5848]. The electrodes are obtained:                by coating an SS (stainless steel) substrate with an aqueous ink comprising CNTs and a dodecylbenzenesulphonate surfactant in order to cover the SS substrate with a film of carbon nanotubes (SS/CNT composite),        by coating the free surface of said film of carbon nanotubes with a mixture comprising LTO or LCO, Super P carbon and a PVdF polymer binder in NMP (N-methyl-2-pyrrolidone) organic solvent, to obtain the composite (SS/CNT/LTO or SS/CNT/LCO), and        immersing said SS/CNT/LTO or SS/CNT/LCO composite in deionized water for easy removal of the CNT/LTO (anode) or CNT/LCO (cathode) bilayer from the SS substrate.        
Before assembling the electrodes with the separator to form a flat battery, the CNT/LTO and CNT/LCO electrodes are cut to the desired format (FIG. S5, “Supporting information” section). However, this cutting step to obtain variable shapes of batteries causes a considerable loss of material, making the coating process too expensive. Moreover, this method for manufacturing electrodes uses compounds (synthetic polymer binder, organic solvent, surfactants) that are not very environment-friendly. In addition, the substrate used for preparing the bilayer electrodes is removed and is not recycled during battery manufacture, as separator for example. Now, the current trend is, in contrast, to find production techniques that have the least possible impact environmentally and obtain devices/batteries that are easily recyclable. Finally, the layer of carbon nanotubes (CNTs) performs the role of current collector. All the half-cell tests are carried out with the bilayer electrode, i.e. in the presence of a current collector. This makes it possible to obtain electrochemical performance that is greatly improved (specific capacity of 147 mAh/g for CNT/LTO) relative to half-cell tests without the current collector (without CNTs).
Faced with the growing demand for autonomous energy sources for applications requiring the production of lithium-ion accumulators with varied and innovative architectures, methods for printing electrodes, in order to produce electrode patterns on demand, have recently been proposed. In particular, patent application FR 2 965 107 A1 proposes preparation of an aqueous ink for making electrodes by printing, comprising at least one electrode active material and at least one water-soluble or water-dispersible conductive polymer such as the combination PEDOT/PSS (poly(3,4-ethylenedioxythiophene)/sodium poly(styrene sulphonate)). The ink is deposited by printing on a metallic current collector. However, such a method has the drawback that it uses a very corrosive binder, and the electrochemical performance of the electrodes prepared by printing is not described.